The present invention relates to an apparatus for two dimensionally coding an image signal.
In an image signal processing system, such as facsimile or an image signal filing system using an optical disc, a picture is composed of a great many picture elements. These picture elements are sorted into black and white to obtain a binary image signal. To reduce the amount of the image signal to be processed, the binary image signal is subjected to coding for reducing the redundancy in the image signal. The redundancy reduction coding is based on the nature of the picture and its correlated image signal. A typical example of the coding is a modified READ coding system, which has been employed in a so-called high speed Group III Standard facsimile. This coding system is a two dimensional successive coding system constructed on the basis of the correlated image signal in both vertical and horizontal directions of the picture. Specifically, in this coding system, the coding is based on positions of picture elements where their density (black or white) changes. In other words, what is coded is not the picture elements per se, but the position data of the picture elements. In the image signal processing system based on the modified READ coding, if there is an error in a code signal during transmission, the influence of the transmission error is successively propagated to the successive scanning lines (horizontal scanning lines). To stop the propagation of the transmission error, the two dimensional coding operation is ceased at every K scanning line, and one dimensional coding (modified Huffman coding) is executed in place of the former coding. The interval between the executions of the modified Huffman coding is defined as the K factor. The K factor is a parameter which must be selected by a designer in the modified READ coding. The modified Huffman coding system is a specific form of the modified READ system, viz., the modified READ system where K=1. Therefore, as the K factor is large, a coding rate (a ratio of the number of bits of an image signal to that of a code signal) is higher in the two dimensional coding system. Actually, the transmission line inevitably contains a predetermined error rate. Therefore, as a coding block (K scanning line) is larger, the code error contained in the code signals of one block transmitted is larger. In Group III Standard, when the code error is detected, the code signal with the code error is not retransmitted. For this reason, in the modified READ system employed in the Group III Standard facsimile, there is a limit to increasing the K factor when the code error caused during the transmission is considered. At present, the K factor is fixed at 2 in a standard resolution mode, and at 4 in a high resolution mode.
Also, when the code signal with the code error is retransmitted, the K factor cannot be made large. Generally, the retransmission is performed at every block. Then, when the K factor is too large, even if the code error is small, the number of bits of the code signal to be retransmitted is large, resulting in reduction of the virtual coding rate. The term "virtual coding rate" means the ratio of the number of bits of an image signal to that of the whole transmitted code signals, including the retransmitted signals, and is called "reduction rate" in this specification.
As described above, in the conventional two dimensional coding system, the K factor is fixed at a small value, depending on the error rate of the transmission line and/or the type of a transmission system. In the recent communication cable which has been modified and made more complicated, it is impossible to make the redundancy reduction rate large with such a small value for the K factor.
Similar problems with facsimile are also true for an image signal processing system such as the image signal filing system. That is, in the image signal filing system, if the code error in transmission is replaced by the writing error at the time of data writing in an optical disc, the image signal filing system has the same problems as those in facsimile.